In the prior art, there are various hydraulic circuits for hydraulically driven vehicles that have been employed mainly for constructional vehicles, work implements and so forth. FIG. 1 shows one example of such a hydraulic circuit for a hydraulically driven vehicle. The shown hydraulic circuit is constructed by connecting discharge output passage 2 of a hydraulic pump 1 to inlet ports 5a and 6a of left and right traveling valves 5 and 6 via left and right pressure compensation valves 3 and 4, connecting outlets of respective left and right traveling valves 5 and 6 to left and right traveling hydraulic motors 7 and 8 and connecting the inlets of the left and right traveling valves 5 and 6 through a by-pass passage 9.
With such a hydraulic circuit, compensation for straight traveling can be done by establishing communication between the inlets of the left and right traveling valves 5 and 6. However, when opening area is differentiated due to tolerance in machining of cut-outs of metering portions of the left and right traveling valves 5 and 6, fluid amounts to be supplied to the respective left and right traveling hydraulic motors 7 and 8 may be differentiated so that precision in compensation for straight traveling can be degraded.
The problem may be solved by connecting a left side fluid line 10 connected to the left side traveling valve 5 and the left side traveling hydraulic motor 7 to a right side fluid line 11 connected to the right side traveling valve 6 and the right side traveling hydraulic motor 8. However, in such construction, since there are respectively two left side fluid lines 10 and right side fluid lines 11, two by-pass passages become necessary, which are disadvantageous in viewpoint of cost and space.